The invention relates to a method and apparatus for suppressing, extinguishing or inhibiting a fire or an explosion in an area.
The term "enclosure" as used in this specification refers to any space having a boundary such as a duct, a cavity, a vessel, a spray dryer, cyclone, silo, fluidizer beds, the hold of a ship, a conveyor, a storage tank, a pump house or the like which may be opened or closed and which may be at any pressure (i.e. above or below atmospheric pressure) or temperature (i.e. above or below ambient temperature).
Various appliances are available to contain or suppress dust explosions in vessels such as dryers, cyclones, connecting duct work, fluidizer beds and powder silos of milk drying plants. All suppression appliances operate on the principle that an explosion is not instantaneous but takes a measurable time, in the order of from 40 to 400 milliseconds to build up to destructive pressure. During a first phase the rate of pressure rise is low, the maximum pressure reaching approximately 1.5 psi. Thereafter the rate of pressure rise rapidly increases, generating up to 100 psi in a second phase. The duration of the pressure rise phases is dependent on the size and geometry of the enclosure in which the explosion occurs. Generally it is recognised that to adequately suppress an explosion the initiating ignition must be suppressed and extinguished within periods of the order of from 10 to 200 milliseconds. To satisfy this requirement the response time of conventional suppression appliances must be very short.
Generally, conventional suppression appliances comprise a detector for detecting the pressure rise caused by an explosion at an early low pressure stage of approximately 0.5 psi. When explosion condition occurs in an enclosure, a control system outputs a signal to burst a diaphragm at the outlet of a suppression charge vessel which introduces a charge of explosion suppressant material into the enclosure. Such suppression systems interrupt particle heat transfer, breaking the combustion chain and preventing rapid pressure rise.
There are three commonly available suppressants in use. These are chlorobromethane (Halon 1011), mono-ammonium phosphate based dry powder (MAP), and water. It has been reported by Moore in The Chemical Engineer, November 1986 and December 1984 that Halon 1011, MAP powder and water are effective in suppressing explosions. The effectiveness of these three different types of suppressants however varies depending on the nature of the explosion. Halon and MAP may contaminate vessels into which they are introduced and this is a considerable disadvantage, particularly in the food industry. Conventional water suppressors have a short period of effectiveness and their use involves a greater risk of reignition.
Somewhat similar comments apply to the extinguishing of fire in any area. "Fire" in this connection refers to a flame front moving at any speed and not only to an explosion which may be characterised as a fast moving fire. The distinction between the terms "fire" and "explosion" is not clearly defined and, where the context allows, the expressions may be interchanged when reading this specification.